Tax Credit Investments - Income Statement Disclosures (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of Income Tax Expense
Affordable housing tax credits and other tax benefits	$ 1,821	$ 1,284
Other tax credit investment credits and tax benefits	5,485	6,286
Total reduction in income tax expense	7,306	7,570
Non-Interest Expenses
Amortization of Tax Credit Investments
Affordable housing tax credit investment losses	986	396
Affordable housing tax credit investment impairment losses	587	124
Other tax credit investment losses	537	767
Other tax credit investment impairment losses	3,164	4,037
Total amortization of tax credit investments recorded in non-interest expense	$ 5,274	$ 5,324